Download Master boot record

Master boot record
http://en.wikipedia.org/wiki/Master_boot_record
A master boot record (MBR) is a type of boot sector, a data sector at the beginning of many types of computer mass storage. It is most common on
disk drives large enough to be partitioned, hence it is not usually present on floppy disks or small thumbdrives.
The master boot record was popularized by the IBM Personal Computer.[1] It consists of 512 bytes located at the first sector of a data storage device
such as a hard disk. MBRs are usually placed on storage devices intended for use with IBM PC-compatible systems.
The MBR may contain one or more of:
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A partition table describing the partitions of a storage device. In this context the boot sector may also be called a partition sector.
Bootstrap code: instructions to identify the configured bootable partition then load and execute its volume boot record
Unique 32-bit disk signature, even though it may never be used by the operating system.[2][3][4][5]
Because of the broad popularity of PC-compatible computers, the MBR format is widely used, to the extent of being supported by computer
operating systems in addition to other pre-existing or cross-platform standards for bootstrapping and partitioning.[6]
Structure of a master boot record
Address
Hex
Oct
Description Size in bytes
Dec
0000
0000
0
01B8
0670
440
code area
disk signature
(optional)
440
(max.
446)
4
01BC
Usually nulls;
0x0000
0674
444
01BE
0676
Table of primary
partitions
(Four 16-byte
446
entries, IBM
partition table
scheme)
01FE
0776
510
2
64
55h
MBR
signature
01FF
0777
511
2
AAh
MBR, total size: 446 + 64 + 2 =
512
IBM PC-DOS introduced the conventional IBM PC (fdisk) partitioning scheme. When a storage device has been partitioned with this scheme, its
MBR contains a partition table describing the locations, sizes, and other attributes of linear regions referred to as partitions.
The MBR is not located in a partition; it is located at a first sector of the device (physical offset 0), preceding the first partition. The boot sector
present on a non-partitioned device or within an individual partition is called a volume boot record. In cases where the computer is running a BIOS
overlay or boot manager, the partition table may be moved to some other physical location on the device; e.g., Ontrack Disk Manager often placed a
copy of the original MBR contents in the second sector, then hid itself from any subsequently booted OS or application, so the MBR copy was
treated as if it were still residing in the first sector.
By convention, there are exactly four primary partition table entries in the MBR partition table scheme, although some operating systems extended
this to five (PTS-DOS with Advanced Active Partitions),[11] eight (AST and NEC MS-DOS)[12][13] or even sixteen (DISKMAN) entries.
http://en.wikipedia.org/wiki/Disk_sector
Disk structures:
(A) Track
(B) Geometrical sector
(C) Track sector
(D) Cluster
Sector: subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user data. Traditional formatting of these
storage media provides space for 512 bytes or 2048 bytes of user-accessible data per sector. Newer hard drives use 4096 byte (4 KB or 4K)
Advanced Format sectors.
Early in the computing industry, the term "block" was loosely used to refer to a small chunk of data. Later the term referring to the data area was
replaced by sector, and block became associated with the data packets that are passed in various sizes by different types of data streams. For example,
the Unix program dd allows one to set the block size to be used during execution with the parameter bs=bytes. This specifies the size of the chunks
of data as delivered by dd and does not affect the size of the sectors used by the medium to which the data is stored.
http://en.wikipedia.org/wiki/Hard_disk_drive_partitioning
Disk partitioning is the act of dividing a hard disk drive into multiple logical storage units referred to as partitions, to treat one physical disk drive as
if it were multiple disks. Partitions are also termed "slices" for operating systems based on BSD, Solaris or GNU Hurd. A partition editor software
program can be used to create, resize, delete, and manipulate these partitions on the hard disk.
Benefits of multiple partitions
Creating more than one partition has the following advantages:
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Separation of the operating system (OS) and program files from user files. This allows image backups (or clones) to be made of only the operating system
and installed software.
Having a separate area for operating system virtual memory swapping/paging.
Keeping frequently used programs and data near each other.
Having cache and log files separate from other files. These can change size dynamically and rapidly, potentially making a file system full.
Use of multi-boot setups, which allow users to have more than one operating system on a single computer. For example, one could install Linux, BSD,
Mac OS X, Microsoft Windows or other operating systems on different partitions of the same hard disk and have a choice of booting into any compatible
operating system at power-up.
Protecting or isolating files, to make it easier to recover a corrupted file system or operating system installation. If one partition is corrupted, other file
systems may not be affected.
Raising overall computer performance on systems where smaller file systems are more efficient. For instance, large hard drives with only one NTFS file
system typically have a very large sequentially accessed Master File Table (MFT) and it generally takes more time to read this MFT than the smaller MFTs
of smaller partitions.
"Short Stroking", which aims to minimize performance-eating head repositioning delays by reducing the number of tracks used per hard drive.[1] The
basic idea is that you make one partition approx. 20-25% of the total size of the drive. This partition is expected to: occupy the outer tracks of the hard
drive, and offer more than double the throughput — less than half the access time. If you limit capacity with short stroking, the minimum throughput
stays much closer to the maximum. This technique, however, is not related to creating multiple partitions, but generally just creating a partition lesser
size than available on the disk.
o For example a 1 TB disk might have an access time of 12 ms at 200 IOPS (at a limited queue depth) with an average throughput of 100 MB/s.
When it is partitioned to 100 GB (and the rest left unallocated) you might end up with an access time of 6 ms at 300 IOPS (with a bigger queue
depth) with an average throughput of 200 MB/s.
[edit] Disadvantages of multiple partitions
Creating more than one partition has the following disadvantages, as compared to having a single partition spanning the same disk area:


Reduces the total space available for user storage on the disk, as it forces the operating system to duplicate certain file system administration areas on
the disk for each partition.
Reduces overall disk performance on systems where data is accessed regularly and in parallel on multiple partitions, because it forces the disk's
read/write head to move back and forth on the disk to access data on each partition[2][3] and to maintain and update file system administration areas on
each partition. It also prevents disk optimizers from moving all frequently accessed files closer to each other on the disk, which could reduce the number
and distance of required head movements. Files can still be moved closer to each other on each partition, but those areas themselves will still be far
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apart on the disk. (See "short stroking" considerations above.) This issue does not apply to Solid-state drives as access times on those are neither
affected by nor dependent upon relative sector positions.
Increases disk fragmentation because it lowers the average size of continuous free blocks on each partition - as compared to a single partition of the
same overall size - after the same amount of data has been written to them.
May prevent using the whole disk capacity, because it may break free capacities apart.[4] For example, if you have a disk with two partitions, each with 3
GBs free (hence 6GBs in total), you can't copy a 4GB DVD image file on that disk, because none of the partitions will actually provide enough space for
that - even though you have more than enough free capacity in total on the disk. If the same files on those two partitions would have been stored on a
single partition spanning the whole disk, then the 4GB file could be easily stored in the 6GB of free space.
Slows down moving data between different parts of the same physical disk.[5] When moving data from one partition to other, the operating system
actually has to copy the contents of the data file, even though it still remains on the same device in a single copy/instance. When using a single partition,
moving data between directories will only require altering the file system administration areas, and the actual contents of the file will not be copied or
moved inside the disk/device, thus resulting in a significantly faster completion of the operation.
Hurts portability and might impose constraints on how entities might be linked together inside the file system. For example: the NTFS file system allows
hard links to be created only as long as both the link and the referenced file reside inside the same volume/partition.[6] Also under Windows if you're
referencing a file on another partition, you can do that only by specifying the partition's assigned drive letter - which, however, might change with time
and depending on the drives installed. This renders references invalid and dependent on actual drive letter assignment, which is not an issue if you have
to reference files/directories only on the same partition, as in this case you can use directory-relative or root-relative references, without including the
drive/partition letter.